The present invention relates to gas turbines having rotational components cooled by a thermal medium flowing within the rotor and particularly relates to thermal medium supply and return tubes extending parallel to the rotor axis adjacent the rim of the rotor for supplying a thermal medium to buckets carried by the turbine wheels and returning spent cooling thermal medium.
In assignee""s prior U.S. Pat. No. 5,593,274, there is disclosed a gas turbine having a closed cooling circuit for supplying a thermal medium, e.g., cooling steam, generally in an axial direction along the rotor to turbine buckets to cool the buckets and returning the spent thermal medium in an opposite, generally axial direction for flow from the rotor, for example, to the steam turbines of a combined-cycle system. In the turbine disclosed in that patent, cooling steam is supplied via an axial bore tube assembly, radially outwardly extending tubes and a plurality of axially extending tubes along the rims of the wheels and spacers for supplying steam to the buckets. Spent cooling steam returns from the buckets through passages in substantially concentric relationship with the cooling steam supply tubes for return via the bore assembly. While such arrangement has proven satisfactory, a new and improved cooling circuit has been designed in connection with a new and further advanced gas turbine.
In accordance with a preferred embodiment of the present invention, the thermal medium, for example, steam, is supplied in an axially forward direction through an aft bore tube assembly, through a plurality of radial tubes in an aft disk, and for flow in supply tubes disposed in aligned openings through the stacked wheels and spacers comprising the rotor and adjacent the rims of the wheels and spacers. The supply tubes lie in communication with the buckets of one or more turbine wheels, preferably the first and second stage buckets, whereby bucket cooling is effected. Spent cooling steam is returned from the buckets via another set of tubes passing in an axial direction through aligned openings adjacent the rims of the wheels and spacers for flow through radially inwardly directed tubes provided in the aft disk for return along the centerline of the bore tube. It has been found highly desirable to minimize the heat lost from the thermal medium flowing through the supply and return tubes into the rotor structure. To accomplish that, the cooling steam is insulated from the rotor structure to minimize the thermal effect on the rotor resulting from the flow of cooling steam through the rotor. Particularly, the tubes are spaced from the walls of the openings to provide insulation between the tubes and the rotor wheels and spacers.
The supply and return tubes also accommodate mechanical and thermal stresses during operation. For example, when the rotor wheels and spacers are assembled, the openings through the wheels and spacers are aligned with one another co-linearly, enabling the tubes to be inserted into the passages defined by the aligned openings after rotor assembly. However, at steady-state turbine operation, the passages do not remain co-linear. Rather, the passages shift out of position relative to one another as a result of mechanical and thermal stresses. Because the masses of the wheels and spacers are different from one another and hence have different mechanical and thermal responses at steady-state, the passages at steady-state turbine operation tend to misalign with one another. Further, the thermal stresses induced by passing cooling steam through the tubes and returning even hotter spent cooling steam causes the tubes to thermally respond, tending to expand the tubes. Additionally, during steady-state operation, the rotor rotates at 3600 rpm. Because the tubes are located about the periphery of the rotor at substantial distances from the rotor axis, substantial centrifugal forces act on the tubes, causing significant stresses in the tubes. With the wheel and spacer passages somewhat misaligned because of the mechanical and thermal stresses on the rotor, the tubes must be designed to minimize any tendency to rupture, crack or become fatigued as a result of lying in a high centrifugal field. Moreover, because the tubes carry cooling steam and are oftentimes during different operational modes at different temperatures than the temperature of the rotor, thermal strain differentials will appear between the tube and rotor which, combined with the centrifugal loading and friction, cause substantial loads on the tubes. If unrestricted, such loads could result in an unpredictable shift in the axial position of the tubes. The axial location of the tubes within the rotor must be constrained within limits to facilitate the flow of steam in different directions relative to the-tubes.
To alleviate or minimize mechanical and thermal stresses on the tubes, the tubes are specifically constructed to have raised lands at axially spaced positions along the tubes separated by thin-walled tube sections. The raised lands thus have exterior surfaces at radial locations larger than the radial locations of the exterior surfaces of the thin-walled sections between the lands. The raised lands engage bushings in the passages through the rotor and, hence, the exterior surfaces of the thin-walled sections are separated by annular spaces from the interior surfaces of the passages. These annular spaces form insulation blankets minimizing the thermal effect of the cooling medium on the rotor.
Transition areas between the lands and the thin-walled sections are also provided to minimize transmission of stresses between the lands and the thin-walled sections. The transition portions include arcuate annular surfaces transitioning from the exterior surface of the lands to the radially reduced exterior surfaces of the thin-walled sections.
Additionally, because the tubes lie in a high centrifugal field during rotor rotation, the heavier the tube, the higher the load applied to tube support bushings. This increased loading on the tube supports increases friction loading as the tubes respond thermally. As the tube responds to the thermal load, the tube grows axially, increasing frictional loading at each support location. The friction load decreases, however, in a direction away from a support which fixes the axial location of the tube in the rotor. By varying the thickness along the tube in accordance with a preferred embodiment of the present invention, and in a direction away from a fixed support for the tube, the load accumulation decreases. Consequently, the thin-walled sections, which are dead weight, can be made progressively thinner in a direction away from the fixed support. That is, the thinner the thin-walled section, the less weight a given support carries and, accordingly, the friction load carried by the tubes decreases as the tube thermally grows. In a preferred form of the invention, the tube is axially fixed adjacent an aft end thereof so that axial tube growth occurs in an axial forward direction. Consequently, the thin-walled sections are increasingly thinner in a direction away from the fixed support, e.g., thinner in an axially forward direction from an aft fixed tube support.
In accordance with another preferred aspect of the present invention, axial retention assemblies are provided on the rotor, preferably on the aft rotor wheel to fix the supply and return tubes at that location, enabling axial thermal growth in an axially forward direction. Each retention assembly, in accordance with a preferred embodiment hereof, includes, for each tube, a pair of retention plates disposed in an annular recess along an annular face of the last wheel of the rotor, e.g., the aft face of the fourth stage wheel in a four-stage turbine. The retention plates are preferably disposed between opposed radial flanges and have arcuate sections straddling the tube extending through the passages and into the annular recess. The tube includes a shoulder against which the retention plate bears to restrain the tube from movement under thermal loading in an axially aft direction. The tube also includes a shoulder for bearing against a portion of the wheel to preclude movement of the tube in an axially forward direction. Slots are preferably formed adjacent the retention plates in the outer flange to facilitate assembly and removal of the retention plates. The retention plates are held in position straddling the tubes by pins engaging in the wheel. Upon removal of the pins, the retention plates can be displaced in a circumferential direction to register radially with slots in the outer flange, enabling the retention plates to be removed from the rotor.
In a preferred embodiment according to the present invention, there is provided multi-stage rotor for a gas turbine, the rotor having an axis, comprising a plurality of turbine wheels and spacers disposed alternately relative to one another along the rotor axis and secured generally in axial alignment with one another, a plurality of axially aligned, circumferentially spaced, openings through the wheels and spacers at locations spaced radially from the axis and tubes disposed in the openings for flowing a thermal medium, the tubes having raised lands at axially spaced locations therealong for mounting the tubes in the passages, the lands having a predetermined wall thickness, the tubes including thin-walled tube sections between the lands of a thickness less than the predetermined thickness and with exterior wall surfaces thereof at radii less than radii of exterior wall surfaces of the lands.
In a further preferred embodiment according to the present invention, there is provided a multi-stage rotor for a turbine, the rotor having an axis, comprising a plurality of turbine wheels and spacers disposed alternately relative to one another along the rotor axis and secured generally in axial alignment with one another, a plurality of axially aligned, circumferentially spaced, openings through the wheels and spacers at locations spaced radially from the axis, tubes disposed in the openings for flowing a thermal medium and a retention plate carried by the rotor for fixing each tube to the rotor against axial displacement in one axial direction and located at a predetermined axial position along the tube, each tube including a shoulder for engaging the plate to preclude displacement of the tube in the one axial direction.